vfs cache: return ENOTDIR for not_a_dir/{.,..} lookups
Reported by: Oliver Kiddle
PR: 270419
MFC: 3 days
Approved by: re (gjb)
(cherry picked from commit c16c4ea6d399332d4a023df6e423be61fdb875a5)
(cherry picked from commit 5d83656c09f522d6bf4f1050be60f270ddef04af)